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1. Efficient all-dielectric metasurface in-coupler for waveguide-based near-eye display

   https://doi.org/10.1117/12.2646494  

   Xiong, Pei; Goodsell, Jeremy; Nikolov, Daniel K; Rolland, Jannick P; Vamivakas, A Nick (March 2023, SPIE)

   Kress, Bernard C; Peroz, Christophe (Ed.)

   The creation of new see-through near-eye displays (NEDs) architectures is a topic of intense research focus. A fundamental problem that each design must address is the field of view (FOV) and eyebox of NEDs are limited by etendue conservation for a fixed display optics size. Waveguide architecture provides the solution to increasing the eyebox in NEDs without increasing the optics size through exit pupil expansion. Brightness and uniformity are two key features of waveguide architecture. In this work, we focus on the brightness of the waveguide since the image uniformity can be compensated by the display engine. We show that the geometry of the waveguide sets a fundamental limit on the in-coupling efficiency for a given FOV. This limit can be used as a tool for waveguide designers to benchmark the in-coupling efficiency of their incoupler gratings. With this derived limit, we designed and optimized a metasurface-based grating (metagrating) and a surface relief grating (SRG) as in-couplers. The diffractive efficiencies of the two types of in-couplers were then compared to the theoretical efficiency limit. The metagrating's 28% efficiency surpasses the SRG's 20% efficiency and nearly matches the geometry-based limit of 29% due to the superior angular response control of metasurfaces compared to SRGs. This work provides a new understanding of the brightness efficiency limit of waveguide-based combiners and paves a novel path toward implementing metasurfaces in efficient waveguide AR displays. 

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2. Metagrating meets the geometry-based efficiency limit for AR waveguide in-couplers

   https://doi.org/10.1364/OE.480092  

   Goodsell, Jeremy; Xiong, Pei; Nikolov, Daniel K.; Vamivakas, A. Nick.; Rolland, Jannick P. (January 2023, Optics Express)

   Recently, augmented reality (AR) displays have attracted considerable attention due to the highly immersive and realistic viewer experience they can provide. One key challenge of AR displays is the fundamental trade-off between the extent of the field-of-view (FOV) and the size of the eyebox, set by the conservation of etendue sets this trade-off. Exit-pupil expansion (EPE) is one possible solution to this problem. However, it comes at the cost of distributing light over a larger area, decreasing the overall system's brightness. In this work, we show that the geometry of the waveguide and the in-coupler sets a fundamental limit on how efficient the combiner can be for a given FOV. This limit can be used as a tool for waveguide designers to benchmark the in-coupling efficiency of their in-coupler gratings. We design a metasurface-based grating (metagrating) and a commonly used SRG as in-couplers using the derived limit to guide optimization. We then compare the diffractive efficiencies of the two types of in-couplers to the theoretical efficiency limit. For our chosen waveguide geometry, the metagrating's 28% efficiency surpasses the SRG's 20% efficiency and nearly matches the geometry-based limit of 29% due to the superior angular response control of metasurfaces compared to SRGs. This work provides new insight into the efficiency limit of waveguide-based combiners and paves a novel path toward implementing metasurfaces in efficient waveguide AR displays. 

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3. Towards Eyeglass-style Holographic Near-eye Displays with Static Expanded Eyebox

   https://doi.org/10.1109/ISMAR50242.2020.00057  

   Xia, Xinxing; Guan, Yunqing; State, Andrei; Chakravarthula, Praneeth; Cham, Tat-Jen; Fuchs, Henry (November 2020, 2020 IEEE International Symposium on Mixed and Augmented Reality (ISMAR))

   null (Ed.)

   Holography is perhaps the only method demonstrated so far that can achieve a wide field of view (FOV) and a compact eyeglass-style form factor for augmented reality (AR) near-eye displays (NEDs). Unfortunately, the eyebox of such NEDs is impractically small ($$\sim \lt$$ 1 mm). In this paper, we introduce and demonstrate a design for holographic NEDs with a practical, wide eyebox of $$\sim$$ 10 mm and without any moving parts, based on holographic lenslets. In our design, a holographic optical element (HOE) based on a lenslet array was fabricated as the image combiner with expanded eyebox. A phase spatial light modulator (SLM) alters the phase of the incident laser light projected onto the HOE combiner such that the virtual image can be perceived at different focus distances, which can reduce the vergence-accommodation conflict (VAC). We have successfully implemented a bench-top prototype following the proposed design. The experimental results show effective eyebox expansion to a size of $$\sim$$ 10 mm. With further work, we hope that these design concepts can be incorporated into eyeglass-size NEDs. 

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4. Triangular cross-section grating couplers for integrated quantum nanophotonic hardware in silicon carbide

   https://doi.org/10.1063/5.0244038  

   Saha, Pranta; Rubin, Alex H; Majety, Sridhar; Dhuey, Scott; Radulaski, Marina (July 2025, APL Materials)

   We design, fabricate, and characterize fishbone grating couplers for triangular cross-section photonics in silicon carbide compatible with color center integration. The periodic and aperiodic grating coupler designs are optimized to outcouple up to 31% of light in the fundamental TE mode of a triangular waveguide. The devices are fabricated using an ion beam etching process in a 4H–SiC sample implanted with NV center ensembles. The room-temperature transmission and the cryogenic NV center photoluminescence collection measurements indicate experimental grating coupler efficiency of up to 24%. This result provides a scalable method to efficiently extract color center light from SiC quantum nanophotonic devices to free-space optics. 

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5. Spatiotemporal reservoir resampling for real-time ray tracing with dynamic direct lighting

   https://doi.org/10.1145/3386569.3392481  

   Bitterli, Benedikt; Wyman, Chris; Pharr, Matt; Shirley, Peter; Lefohn, Aaron; Jarosz, Wojciech (July 2020, ACM transactions on graphics)

   Efficiently rendering direct lighting from millions of dynamic light sources using Monte Carlo integration remains a challenging problem, even for off-line rendering systems. We introduce a new algorithm—ReSTIR—that renders such lighting interactively, at high quality, and without needing to maintain complex data structures. We repeatedly resample a set of candidate light samples and apply further spatial and temporal resampling to leverage information from relevant nearby samples. We derive an unbiased Monte Carlo estimator for this approach, and show that it achieves equal-error 6×-60× faster than state-of-the-art methods. A biased estimator reduces noise further and is 35×-65× faster, at the cost of some energy loss. We implemented our approach on the GPU, rendering complex scenes containing up to 3.4 million dynamic, emissive triangles in under 50 ms per frame while tracing at most 8 rays per pixel. 

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